This invention relates to sensors for detecting a gas in a test medium. More particularly, this invention relates to sensors that use electromagnetic absorption spectroscopy or fluorescence quenching spectroscopy for measuring the concentration of the gas in the test medium.
Using optical absorption measurements to find the concentration of a gas in a test medium is a method well known in the prior art. When radiation with a spectrum of wavelengths passes through a gas, a profile of absorption bands or the absorption spectrum of the gas can be obtained. Since each gas has its specific absorption spectrum, the type and quantity of the gas present in a sample can thus be specified via its absorption spectrum.
The medical profession has used gas sensors to monitor data such as gases in the blood of a surgical patient. Various medical personnel can track and evaluate the patient's metabolism and respiratory effectiveness from such data. There are prior art optical absorption gas sensors that are small enough to fit inside a catheter. This combination when inserted by an operator into a blood vessel permits continuous and instantaneous blood gas measurements.
The typical blood-gas sensors measure the optical absorption of the gas in the blood. The lowest gas concentration that can be detected in a test medium is the concentration that gives a detectable signal higher than the noise of the measuring system. Some carbon dioxide (CO.sub.2) sensors operate at the strongly absorbed wavelengths around 4 .mu.m (microns) for increased sensitivity. This entails the use of complex and expensive optical components to generate, filter, and measure these wavelengths. Making remote CO.sub.2 measurements requires expensive optical fibers that have low absorption at these wavelengths in order to avoid interference with the measurement. Besides being excessively brittle for catheter use, these fibers have known reactivity with water that can compromise patient safety.
The medical profession has used commercially available gas sensors such as a Capnometer to measure the concentration of CO.sub.2 in the airway stream of a patient. An operator mounts the CO.sub.2 sensor on a breathing ventilator or breathing tube for monitoring the effectiveness of patient's ventilation. The sensor in the Hewlett Packard (HP) Capnometer (Model 47210) weighs several ounces, making it too heavy to be mounted on the airway tube of a neonate. The Novametrix Model 1260 Capnograph employs a sensor that weighs less than 1 ounce. However, in order to use this sensor with a neonate, that patient still must be intubated, as is the case for the HP instrument.
Other known airway devices operating at wavelengths around 4 .mu.m typically use a heated light source to generate these wavelengths. This type of light source consumes a significant amount of electrical power and generates heat. This heat produces undesirable temperature gradients which adversely affect the measurement. Finally, many of these instruments are too inconveniently heavy to perch atop a breathing tube.
In a prior art thin film gas sensor, the refractive index of the measuring medium is very different from that of the sensing path capturing the optical radiation. The radiation used for detecting the concentration of the selected gas couples into the boundary layers of the measuring medium through total internal reflection. Since the radiation only reacts with the gas in the boundary layer of the measuring medium, a sufficiently long path is required to give a measurably accurate result. Moreover, this prior art device uses fluoride-glass fibers which, as mentioned previously, are expensive, excessively brittle and reactive when used in contact with moisture-bearing gas streams.
In summary, there is a need for a new gas sensor operating at wavelengths shorter than 4 .mu.m in order to reduce the complexity and cost of sensor components. Furthermore, it is desirable for that new sensor to be more rugged, lightweight and compact than prior art gas sensors.